First, on one machine, go to System Preferences > Sharing, and turn File Sharing on. Then go into the Network preference pane, and note the IP of the computer on whichever network connector you're using (Ethernet, AirPort).
Then, on the other machine, in Finder, choose "Go > Connect to Server…" or Cmd+K, and type in afp://the.ip.you.noted (e.g. afp://192.168.0.2) and click Connect.
Then type in the user name and password.
You might have luck by "browsing" the network, but I find that that's a bit flaky. This way will work no problems, assuming the computers are networked.